Generally speaking, such a stator comprises:
a magnetic circuit constituted by an axial succession of rings made up of magnetic laminations and disposed co-axially about said longitudinal axis;
conductor windings extending axially in grooves made in the inside cylindrical surface of the magnetic circuit;
assembly rods spaced angularly around and extending longitudinally along the cylindrical outside surface of the magnetic circuit, with a radially inner portion of each rod being dove-tailed into each ring of the magnetic circuit;
axial tightening means for axially tightening the magnetic circuit;
a casing disposed coaxially about the stator outside the magnetic circuit, said casing comprising an outer cylindrical sheath with annular stiffening rings axially spaced along the inside of the sheath and welded thereto; and
legs for fixing the casing to a support structure external to the stator.
In normal operation, the magnetic circuit of the stator in a large electrical machine is deformed by the forces which are induced by the magnetic field of the rotor. Each point on the outer cylindrical surface of the magnetic circuit moves synchronously with the rotation of the rotor, with a motion which is the resultant of a radial oscillation and a tangential oscillation. More precisely, each point moves substantially around an ellipse whose long axis is the amplitude of the radial component and whose short axis is the amplitude of the tangential component.
If these oscillations are transmitted undamped to the stator casing and hence to its foundations, they give rise to noise and to fatigue failures in the connecting parts subjected to the oscillations.
It is thus common practice to suspend the stator magnetic circuit from resilient means which provide the mechanical connection between the stator and the foundations on which the alternator is built, and which also provide effective damping of the vibration generated by large electrical machines before transmitting damped vibrations to the casing and thence to the foundations.
In a first known type of resilient suspension, the magnetic circuit is mounted in an inner casing or corset which is generally cylindrical in shape, and the resilient members are fixed along two or three generator lines of the corset (e.g. one on either side and one underneath). The resilient members are in the form of spring blades and they are positioned tangentially to the magnetic circuit and to the machine axis in such a manner as to provide a connection between the corset and the outer casing of the alternator.
This first known type has the drawbacks of requiring two casings to be constructed, which is costly and increases the bulk of the stator.
A second known type of resilient suspension includes blade springs extending axially between some of the assembly rods of the magnetic circuit and the stiffening rings of the casing. Each blade has axially separated fixing points therealong, fixing the spring alternately to the fixing rod and to the stiffening rings of the casing. The springs connect the magnetic circuit to the casing and damp the radial and tangential displacements of the magnetic circuit.
The spring blades are bunched in the most rigid zones of the casing, i.e. the zones which are least likely move because of elastic deformation of the casing. These zones are the zones which are closest to the legs for fixing the casing to an outside block of concrete.
This second type of known resilient suspension system is described in U.S. Pat. No. 4,145,626 (corresponding to French specification No. 2 376 542). The springs described are formed by having some of the assembly rods extend radially outwardly with slots dividing them over some portions of their length into inner portions which constitute a magnetic circuit assembly rod per se, and radially outer portions acting a spring blades. Such a system is relatively easy to manufacture, but it does not lend itself to obtaining adequate mechanical resiliance and damping of radial and tangential displacements.
Finally, in either of these known types of resilient suspension system, the system is subjected not only the vibrations due to normal operation, but also to loads and forces due to: the mass of the magnetic circuit and its windings; nominal operating torque; and, on occasion, the large torque which may be generated by an accidental electrical fault (short circuit, wrong coupling,). The need to be able to withstand all these forces and loads reliably makes such a system expensive to put into practice.
Further, U.S. Pat. No. 2,424,299 describes a stator for an electrical machine comprising: floating stiffening rings, fixed stiffening rings, assembly rods, and suspension rods shaped such that their central portions are radially resilient while being substantially stiff in the axial direction. However, the stiffening rings described do not include any means for limiting the kind of tangential displacement to be expected of the magnetic circuit in the event of an electrical short circuit for example, and therefore run the risk of damage to the mechanical and the electrical parts of the machine.
Preferred embodiments of the present invention provide simple and cheap means for resiliently suspending the stator of a synchronous dynamo-electrical machine, which means are capable of effectively damping vibrations due to normal operation without increasing the bulk of the stator and without running the risk of tangential displacement of the magnetic circuit in the event of a fault such as a short circuit across the machine's terminals.